Display device

ABSTRACT

Provided is a display device containing quantum dots. A display device includes a display area. The display area has a light emitting device in which a first electrode, a layer between the first electrode and an emitting layer, the emitting layer, a layer between the emitting layer and a second electrode, and the second electrode are stacked in this order on a substrate. The emitting layer is formed of an inorganic layer containing quantum dots, and the light emitting device is a top emission device. A thin film transistor connected to the light emitting device is preferably an n-ch TFT.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a U.S. National Stage Entry of International Patent ApplicationNo. PCT/JP2018/041260, filed Nov. 7, 2018, which claims priority toJapanese Patent Application No. 2017-215801, filed Nov. 8, 2017.

TECHNICAL FIELD

The present invention relates to a display device using quantum dots.

BACKGROUND ART

JP 2017-045650 A (PTL 1) discloses an invention relating to organicelectro-luminescence (EL).

An organic EL device has a structure in which an anode, a hole injectionlayer, a hole transport layer, an emitting layer, an electron transportlayer, an electron injection layer, and a cathode are stacked on asubstrate.

Such an organic EL device is formed from an organic compound and emitslight from excitons formed by the recombination of electrons and holesinjected into the organic compound.

CITATION LIST Patent Literature

-   PTL 1: JP 2017-045650 A

SUMMARY OF INVENTION Technical Problem

In recent years, light emitting devices using quantum dots are beingdeveloped. Quantum dots are nanoparticles made of around severalhundreds to several thousands of atoms, each having a particle diameterof around several nanometers to several tens of nanometers. Quantum dotsare also referred to as fluorescent nanoparticles, semiconductornanoparticles, or nanocrystals. The emission wavelength of quantum dotsmay be variously changed depending on the particle diameter and thecomposition of the nanoparticles. As with an organic EL device, a lightemitting device using quantum dots makes it possible to obtain a thinnerdevice and surface emission.

However, the layered structure of top emission light emitting devicesusing quantum dots, and the structure of display devices using suchlight emitting devices have not yet been established.

The present invention is made in consideration of the above, and seeksto provide a display device having a light emitting device that includesquantum dots.

Solution to Problem

A display device according to the present invention includes a displayarea. The display area has a light emitting device in which a firstelectrode, a layer between the first electrode and an emitting layer,the emitting layer, a layer between the emitting layer and a secondelectrode, and the second electrode are stacked in this order on asubstrate. The emitting layer is formed of an inorganic layer containingquantum dots, and the light emitting device is a top emission device.

In an aspect of the present invention, a thin film transistor connectedto the light emitting device is preferably an n-ch TFT.

In another aspect of the present invention, an oxide semiconductor ofthe thin film transistor is preferably an In—Ga—Zn—O-basedsemiconductor.

In yet another aspect of the present invention, the display device ispreferably flexible.

In yet another aspect of the present invention, the quantum dotspreferably have a structure in which a surface of a core is not coveredby a shell.

In yet another aspect of the present invention, at least one of thelayer between the first electrode and the emitting layer, the emittinglayer, and the layer between the emitting layer and the second electrodeis formed by an inkjet process.

In yet another aspect of the present invention, the layer between thefirst electrode and the emitting layer, and the emitting layer arepreferably formed by coating; and the layer between the emitting layerand the second electrode is preferably formed by vapor deposition orcoating.

Advantageous Effects of Invention

In a display device of the present invention, the layered structure oflight emitting devices containing quantum dots, used in a display devicecan be optimized. Further, in the present invention, all the layers fromthe cathode to the anode can be formed of inorganic layers.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 is a partial plan view of a display device according to oneembodiment;

FIG. 2 is a partial enlarged cross-sectional view illustrating one ofdisplay areas of the display device depicted in FIG. 1 that is enlarged;

FIG. 3 is a cross-sectional view illustrating the structure of a thinfilm transistor different from one in FIG. 2 ;

FIG. 4A is a cross-sectional view of a light emitting device ofEmbodiment 1, and FIG. 4B is an energy level diagram of each layer in adisplay device of Embodiment 1;

FIGS. 5A and 5B are schematic views of a quantum dot according to oneembodiment;

FIGS. 6A to 6C are cross-sectional views each illustrating a lightemitting device according to an embodiment different from that in FIG. 1;

FIG. 7A is an energy level diagram in the case of using quantum dotshaving a core-shell structure, and FIG. 7B is an energy level diagram inthe case of using quantum dots having a structure in which a core is notcovered by a shell;

FIG. 8A is a cross-sectional view of a light emitting device differentfrom one in FIG. 4 , and FIG. 8B is an energy level diagram of eachlayer in the light emitting device of FIG. 8A;

FIGS. 9A to 9C are cross-sectional views each illustrating a lightemitting device according to an embodiment different from that in FIG. 8;

FIG. 10A is an energy level diagram in the case of using quantum dotshaving a core-shell structure, and FIG. 10B is an energy level diagramin the case of using quantum dots having a structure in which a core isnot covered by a shell;

FIG. 11 is a schematic view illustrating a step of forming an inorganiclayer by the inkjet process;

FIG. 12 is a photograph showing an application in Examples;

FIG. 13 shows PYS measurement data of Cd-based green quantum dots;

FIG. 14 shows PYS measurement data;

FIG. 15 is an energy level diagram of each layer in the light emittingdevice used in an experiment;

FIG. 16 is a graph illustrating the relationship between the currentvalue and the EQE of an EL emitter and a PL emitter using green quantumdots;

FIG. 17 shows plots illustrating the relationship between the currentvalue and the EQE of an EL emitter and a PL emitter using red quantumdots; and a plot illustrating the relationship between the current valueand the EQE of an EL emitter using blue quantum dots;

FIG. 18 presents a graph illustrating the energy band gap Eg of eachlayer in the light emitting device used in an experiment, the energyE_(CB) at the bottom of the conduction band, and the energy E_(CB) atthe top of the conduction band, and an energy level diagram of thelayers;

FIG. 19 shows the UV data of ZnO_(X)(Li) and ZnO_(X)(K) used in anelectron transport layer (EU);

FIG. 20 shows the PL data of ZnO_(X)(Li) and ZnO_(X)(K) used in anelectron transport layer (ELL); and

FIG. 21 shows the PYS data of ZnO_(X)(Li) and ZnO_(X)(K) used in anelectron transport layer (ETL).

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention (hereinafter simply referred to as“embodiments”) will now be described in detail. Note that the presentinvention is not limited to the following embodiments, and variousmodifications may be made without departing from the spirit of thepresent invention.

As illustrated in FIG. 1 , a plurality of display areas 2 are arrangedin a matrix in the display device 1. The display areas 2 fall into threetypes: red emission regions 2 a emitting red light, green emissionregions 2 b emitting green light, and blue emission regions 2 c emittingblue light. These three emission regions 2 a, 2 b, and 2 c are forexample arranged in a row direction to form a set constituting one pixelfor color display.

In each of the emission regions 2 a, 2 b, and 2 c, a light emittingdevice 3 is formed. The layer structure of the light emitting device 3will be described below. A thin film transistor (TFT) 4 is connected toeach light emitting device 3. The light emitting devices 3 are topemission devices.

The thin film transistor 4 illustrated in FIG. 2 is an n-ch TFT, whichis built in such a manner that a gate electrode 4 a, a channel layer 4b, a gate insulating film (not shown), a drain electrode 4 c, a sourceelectrode 4 d, etc. are stacked on a substrate 5. The channel layer 4 bis formed from an N-type semiconductor; preferably, an oxidesemiconductor is used. However, the material of the layer is not limitedto this. As the oxide semiconductor, an In—Ga—Zn—O-based semiconductoris preferably used. An In—Ga—Zn—O-based semiconductor has high mobilityand causes low leakage current, and thus can be suitably used in thethin film transistor. Alternatively, Poly-Si can be preferably used. Thethin film transistor 4 depicted in FIG. 2 has a top-contact bottom-gateconfiguration; alternatively, the thin film transistor 4 may have abottom-contact bottom-gate configuration.

The source electrode 4 d is connected to a power supply line, and thedrain electrode 4 c is connected to the light emitting device 3.

Further, the thin film transistor 4 may have a top-gate configurationdepicted in FIG. 3 . As illustrated in FIG. 3 , the channel layer 4 b isformed on the substrate 5, and the surface of the channel layer 4 iscovered with the gate insulating film 4 e. The gate electrode 4 a isformed on the surface of the gate insulating film 4 e. As illustrated inFIG. 3 , the surface of the gate electrode 4 a is covered with aninsulating film 4 f. Further, a plurality of through holes extendingthrough the gate insulating film 4 e and the insulating film 4 f toreach the channel layer 4 b, and the drain electrode 4 c and the sourceelectrode 4 d are formed in the respective through holes. Further, thesurfaces of the drain electrode 4 c and the source electrode 4 d arecovered with a protective film 7. Further, a transparent electrodeconnected to the drain electrode 4 c and the source electrode 4 d isformed on the surface of the protective film 7. The transparentelectrode 8 depicted in FIG. 3 is connected to the drain electrode 4 c.

The channel layer 4 b of the thin film transistor 4 shown in FIG. 3 isan N-type semiconductor, and an oxide semiconductor is preferably used.As the oxide semiconductor, an In—Ga—Zn—O-based semiconductor ispreferably used.

As illustrated in FIG. 2 , the display device 1 has a structure in whichthe thin film transistor 4 and the light emitting device 3 areinterposed between a pair of substrates 5 and 6, and, although notshown, sealing resin is provided in a frame pattern between thesubstrates 5 and 6 so that the substrates 5 and 6 are connected with thesealing resin therebetween.

Hereinafter, the structure of the light emitting device 3 will bedescribed. FIG. 4A is a cross-sectional view of a light emitting deviceof Embodiment 1, and FIG. 4B is an energy level diagram of each layer ina display device of Embodiment 1.

As shown in FIG. 4A, the light emitting device 3 is configured to have asubstrate 10, a cathode 15 formed on the substrate, an electrontransport layer (ETL) 14 formed on the cathode 15, an emitting layer(EML) 13 formed on the electron transport layer 14, a hole transportlayer (HTL) 12 formed on the emitting layer 13, and an anode 11 formedon the hole transport layer 12.

When a voltage is applied to between the electrodes of the lightemitting device 3 in this embodiment, holes are injected from the anode11, and electrons are injected from the cathode 15. FIG. 4B shows theenergy level models of the hole transport layer 12, the emitting layer13, and the electron transport layer 14. As shown in FIG. 4B, holestransported through the hole transport layer 12 are injected from theHOMO level of the hole transport layer 12 into the HOMO level of theemitting layer 13. On the other hand, electrons transported through theelectron transport layer 14 are injected from the LUMO level of theelectron transport layer 14 into the LUMO level of the emitting layer13. The holes and electrons are recombined in the emitting layer 13,which promotes quantum dots in the emitting layer 13 to the excitedstate, thus light emission from the excited quantum dots can beachieved.

In this embodiment, the emitting layer 13 is formed of an inorganiclayer containing quantum dots.

(Quantum Dot)

For example, quantum dots in this embodiment are nanoparticles having aparticle diameter of around several nanometers to several tens ofnanometers; however, the structure and the material of the quantum dotsare not limited to those.

For example, quantum dots are formed from CdS, CdSe, ZnS, ZnSe, ZnSeS,ZnTe, ZnTeS, InP, (Zn)AgInS₂, (Zn)CuInS₂, etc. Because of the toxicityof Cd, the use of Cd is restricted in many countries; thus, quantum dotsare preferably free of Cd.

As shown in FIG. 5A, many organic ligands 21 are preferably placed onthe surface of a quantum dot 20. This can inhibit aggregation of quantumdots 20, resulting in the target optical properties. The ligandsavailable for the reaction are not particularly limited; for example,the following ligands can be given as typical examples.

Aliphatic primary amines: oleylamine: C₁₈H₃₅NH₂,stearyl(octadecyl)amine: C₁₈H₃₇NH₂, dodecyl(lauryl)amine: C₁₂H₂₅NH₂,decylamine: C₁₀H₂₁NH₂, octylamine: C₈H₁₇NH₂

Aliphatic acids: oleic acid: C₁₇H₃₃COOH, stearic acid: C₁₇H₃₅COOH,palmitic acid: C₁₅H₃₁COOH, myristic acid: C₁₃H₂₇COOH, lauric(dodecanoic) acid: C₁₁H₂₃COOH, decanoic acid: C₉H₁₉COOH, octanoic acid:C₇H₁₅COOH

Thiols: octadecanethiol: C₁₈H₃₇SH, hexadecanethiol: C₁₆H₃₃SH,tetradecanethiol: C₁₄H₂₉SH, dodecanethiol: C₁₂H₂₅SH, decanethiol:C₁₀H₂₁SH, octanethiol: C₈H₁₇SH

Phosphines: trioctylphosphine: (C₈H₁₇)₃P, triphenylphosphine: (C₆H₅)₃P,tributylphosphine: (C₄H₉)₃P

Phosphine oxides: trioctylphosphine oxide: (C₈H₁₇)₃P═O,triphenylphosphine oxide: (C₆H₅)₃P═O, tributylphosphine oxide:(C₄H₉)₃P═O

A quantum dot 20 depicted in FIG. 5B has a core-shell structure having acore 20 a and a shell 20 b covering the surface of the core 20 a. Asshown in FIG. 5B, many organic ligands 21 are preferably placed on thesurface of the quantum dot 20. The core 20 a of the quantum dot 20 shownin FIG. 5B is the nanoparticle shown in FIG. 5A. Accordingly, the core20 a is formed for example from the materials listed above. The shell 20b is formed from, for example, zinc sulfide (ZnS); however, the materialof the shell 20 b is not limited to this. As with the core 20 a, theshell 20 b is preferably free of cadmium (Cd).

The shell 20 b may be in a condition of being a solid solution on thesurface of the core 20 a. In FIG. 5B, the boundary between the core 20 aand the shell 20 b is indicated by a dotted line, and this means thatthe boundary between the core 20 a and the shell 20 b may or may not beidentified by an analysis.

(Emitting Layer 13)

The emitting layer 13 may be formed from the above-mentioned quantumdots alone; alternatively, the emitting layer 13 may contain the quantumdots and another luminescent material other than the quantum dots.Further, the emitting layer 13 may be formed by applying quantum dotsdissolved in a solvent, for example, by the inkjet process. Here, aslight amount of the solvent component may be left in the emitting layer13.

Red quantum dots emitting red light are contained in the emitting layer13 of the light emitting devices 3 formed in the red emission regions 2a depicted in FIG. 1 . Further, green quantum dots emitting green lightare contained in the emitting layer 13 in the light emitting devices 3formed in the green emission regions 2 b depicted in FIG. 1 . Further,blue quantum dots emitting blue light are contained in the emittinglayer 13 in the light emitting devices 3 formed in the blue emissionregions 2 c depicted in FIG. 1 .

Note that the wavelength of blue emission is preferably around 450 nm.Thus, health risks can be reduced by adjustments such that light of awavelength shorter than 450 nm is not emitted can.

The emitting layer 13 can be formed by an existing thin film formationmethod such as the inkjet process and vacuum deposition mentioned above.

(Hole Transport Layer 12)

The hole transport layer 12 is made from an inorganic material or anorganic material having hole transporting functions. The hole transportlayer 12 is preferably made from an inorganic material, for example, ispreferably formed from an inorganic oxide such as NiO or WO₃. Inparticular, the hole transport layer 12 is preferably formed fromnanoparticles of NiO. Further, for use in the hole transport layer 12,for example, Al₂O₃ or the like may be mixed in NiO. And, a metal oxidemay be doped with Li, Mg, Al, etc. Further, the hole transport layer 12may be of an inorganic material other than inorganic oxides.

As with the emitting layer 13, the hole transport layer 12 can be formedby a printing process such as the inkjet process, or may be formed by anexisting thin film technique such as vacuum deposition.

(Electron Transport Layer 14)

The electron transport layer 14 is made from an inorganic material or anorganic material having electron transporting functions. The electrontransport layer 14 is preferably made from an inorganic material, forexample, is preferably formed from an inorganic oxide such as ZnO_(X),Ti—O, Sn—O, V-O_(X), or Mo—O. Two or more of these materials may beselected as materials. In particular, the electron transport layer 14 ispreferably formed from nanoparticles of ZnO_(X). And, a metal oxide maybe doped with Li, Mg, Al, Mn, etc. Further, the electron transport layer14 may be of an inorganic material (for example, CsPbBr₃ etc.) otherthan inorganic oxides. X is, but not limited to, around 0.8 to 1.2.

As with the emitting layer 13, the electron transport layer 14 can beformed by applying a solvent containing nanoparticles by a printingprocess such as the inkjet process, or may be formed by an existing thinfilm technique such as vacuum deposition.

(Anode 11)

In this embodiment, the material for forming the anode 11 is preferablyfor example, but not limited to, a metal such as Au or Ag, a conductivetransparent material such as CuISnO₂ or ZnO_(X), or a compound oxide ofindium-tin (ITO). Of those, the anode 11 is preferably formed from ITO.The anode 11 can be formed as a thin film of the electrode material onthe substrate 10 by a method such as vapor deposition or sputtering.

In this embodiment, since a structure in which light is given off fromthe anode 11 side is used, the anode 11 needs to be transparent, and ispreferably a thin metal film of Ag etc. that is excellent intransparency, or one of the metal oxides excellent in transparency thatare mentioned above.

(Cathode 15)

In this embodiment, for example, a compound oxide of indium-tin (ITO), ametal, an alloy, an electrically conductive compound, or a mixture ofthose can be used as an electrode material of the cathode 15; however,the material of the cathode 15 is not limited to those. For example, thecathode 15 is formed from ITO. The cathode 15 is, for example, formed onan opaque metal layer formed on the substrate 10. Thus, the lightemitting device 3 is obtained as a top emission device.

The cathode 15 can be formed as a thin film of the electrode material bya method such as vapor deposition or sputtering.

(Substrate 10)

In this embodiment, the substrate 10 can be formed of, for example,glass or plastic; however, the material of the substrate 10 is notlimited to these. Specifically, the substrate 10 is formed of, forexample, glass, quartz, or a transparent resin film.

The substrate 10 may either be a rigid substrate or a flexiblesubstrate; when a flexible substrate is used, a flexible device can beobtained. Examples of the material of the transparent film include, forexample, polyethylene terephthalate (PET), polyethylene naphthalate(PEN), polyester, polyethylene, polypropylene, cellophane, cellulosediacetate, and cellulose triacetate (TAC).

In the display device 1 in FIG. 2 , using flexible substrates for thesubstrates 5 and 6, the display device 1 can be flexible. Note that thesubstrates 5 and 6 can also be formed using a similar material to thatof the substrate 10. The substrate 5 can also serve as the substrate 10.

In this embodiment, all the layers from the cathode 15 to the anode 11,that is, all the cathode 15, the electron transport layer 14, theemitting layer 13, the hole transport layer 12, and the anode 11 caneach be formed of an inorganic layer. Forming all the layers frominorganic layers allow all the layers to be formed using the samecoating/drying apparatuses, etc. and facilitates the production process.Further, the high-low relationships of the HOMO levels of the anode 11,the hole transport layer 12, and the emitting layer 13 can be optimized;the high-low relationships of the LUMO levels of the cathode 15, theelectron transport layer 14, and the emitting layer 13 can be optimized.This improves the carrier balance as compared with the case of usingorganic compounds.

In Embodiment 1 illustrated in FIG. 4 , a hole injection layer and anelectron injection layer are not formed separately from the transportlayers, thus the number of the layers can be reduced. Namely, thetransport layers also serve as injection layers. Note however that inthis embodiment, a hole injection layer and an electron injection layermade of an inorganic material may be interposed between the electrodesand the transport layers.

FIG. 6A is a cross-sectional view of a light emitting device ofEmbodiment 2. In FIG. 6A, the cathode 15, the electron transport layer14, the emitting layer 13, the hole transport layer 12, a hole injectionlayer (HIL) 16, and the anode 11 are stacked in this order on thesubstrate 10. Unlike in FIG. 4A, in FIG. 6A, the hole injection layer 16is placed between the anode 11 and the hole transport layer 12.

FIG. 6B is a cross-sectional view of a light emitting device ofEmbodiment 3. In FIG. 6B, the cathode 15, an electron injection layer(EIL) 18, the electron transport layer 14, the emitting layer 13, thehole transport layer 12, and the anode 11 are stacked in this order onthe substrate 10. Unlike in FIG. 4A, in FIG. 6B, the electron injectionlayer 18 is placed between the electron transport layer 14 and thecathode 15.

FIG. 6C is a cross-sectional view of a light emitting device ofEmbodiment 4. In FIG. 6C, the cathode 15, the electron injection layer18, the electron transport layer 14, the emitting layer 13, the holetransport layer 12, the hole injection layer 16, and the anode 11 arestacked in this order on the substrate 10. Unlike in FIG. 4A, in FIG.6C, the hole injection layer 16 is placed between the anode 11 and thehole transport layer 12, and in addition, the electron injection layer18 is placed between the electron transport layer 14 and the cathode 15.

The material of the hole injection layer 16 and the electron injectionlayer 18 may be either an inorganic material or an organic material.Forming the hole injection layer 16 and the electron injection layer 18from inorganic layers is preferred because all the layers from the anode11 to the cathode 15 can each be formed of an inorganic layer. However,the material of the injection layers is not limited to this. Thematerial of the hole injection layer 16 and the electron injection layer18 is selected from various kinds of materials depending on the energylevel models.

In this embodiment, the layer between the cathode 15 and the lightemitting layer 13 is preferably a layer serving as the electrontransport layer 14, the electron injection layer 18, or both theelectron injection layer and the electron transport layer; or a layer inwhich the electron transport layer 14 and the electron injection layer18 are stacked.

In this embodiment, the layer between the anode 11 and the emittinglayer 13 is preferably a layer serving as the hole transport layer 12,the hole injection layer 16, or both the hole injection layer and thehole transport layer; or a layer in which the hole transport layer 12and the hole injection layer 16 are stacked.

Note that in the structure of the display device 1 depicted in FIG. 2 ,the thin film transistor 4 has, for example, a bottom-gateconfiguration, and the drain electrode 4 c is connected to the cathode15 of the light emitting device 3. Here, the drain electrode 4 c can bemade to serve as the cathode 15 without forming the cathode 15superposed on the drain electrode 4 c. This allows the light emittingdevice 3 to be suitably connected with the thin film transistor 4 andthe ground line.

In this embodiment, the hole transport layer 12, the emitting layer 13,and the electron transport layer 14 can all be inorganic layers formedfrom nanoparticles. In such a case, each layer can be formed by printingby the inkjet process or the like, thus the layers can be formed easilyand formed to be uniform in thickness. This can effectively improveemission efficiency.

When the quantum dots used in the emitting layer 13 of this embodimenthave a core-shell structure, the energy level diagram presented in FIG.7A is obtained, and the energy level of the shell would be as a barrierto the recombination of holes and electrons. Accordingly, a quantum dotof which surface is not covered with a shell (the surface of the core isexposed, or the material forming the quantum dot is uniform from thecenter of the quantum dot to the surface thereof) as shown in FIG. 7B ispreferably used. Using such quantum dots eliminates the energy barrierto the recombination of hole and electrons, and allows holes andelectrons to be efficiently recombined, thus the light emissionefficiency can be improved. With a view to improving the electrontransportation efficiency and the hole transportation efficiency, theorganic ligands 21 are preferably placed on the surface of each quantumdot 20 as illustrated in FIG. 5A.

Further, in this embodiment, as illustrated in FIG. 8A, the lightemitting device 3 may be configured to have the substrate 10, the anode11 formed on the substrate, the hole transport layer (HTL) 12 formed onthe anode 11, the emitting layer (EML) 13 formed on the hole transportlayer 12, the electron transport layer (ETL) 14 formed on the emittinglayer 13, and the cathode 15 formed on the electron transport layer 14.

The materials of the layers are as described above. Note that in FIG.8A, the anode 11 forms a first electrode, and the cathode 15 forms asecond electrode. Since the light emitting device 3 of this embodimentis a top emission device, the cathode 15 is preferably formed of a verythin transparent material such as Ag, and the anode 11 is preferablymade of ITO, for example, on an opaque metal layer. This allows light tobe reflected at the anode 11 and light to be given off from the surfaceside on the cathode 15 side (opposite to the thin film transistor side).

FIG. 9A is a cross-sectional view of a light emitting device accordingto a different embodiment from FIG. 8A. In FIG. 9A, the anode 11, thehole injection layer (HIL) 16, the hole transport layer 12, the emittinglayer 13, the electron transport layer 14, and the cathode 15 arestacked in this order on the substrate 10. Unlike in FIG. 8A, in FIG.9A, the hole injection layer 16 is placed between the anode 11 and thehole transport layer 12.

FIG. 9B is a cross-sectional view of a light emitting device accordingto a different embodiment from FIG. 8A. In FIG. 9B, the anode 11, thehole transport layer 12, the emitting layer 13, the electron transportlayer 14, the electron injection layer (EIL) 18, and the cathode 15 arestacked in this order on the substrate 10. Unlike in FIG. 8A, in FIG.9B, the electron injection layer 18 is placed between the electrontransport layer 14 and the cathode 15.

FIG. 9C is a cross-sectional view of a light emitting device accordingto Embodiment 4. In FIG. 9C, the anode 11, the hole injection layer 16,the hole transport layer 12, the emitting layer 13, the electrontransport layer 14, the electron injection layer 18, and the cathode 15are stacked in this order on the substrate 10. Unlike in FIG. 8A, inFIG. 9C, the hole injection layer 16 is placed between the anode 11 andthe hole transport layer 12, and in addition, the electron injectionlayer 18 is placed between the electron transport layer 14 and thecathode 15.

When the quantum dots used in the emitting layer 13 of the embodimentsin FIG. 8A to FIG. 9C have a core-shell structure, the energy leveldiagram presented in FIG. 10A is obtained, and the energy level of theshell would be as a barrier to the recombination of holes and electrons.Accordingly, using quantum dots in which the surface of the core is notcovered with a shell as depicted in FIG. 10B eliminates the energybarrier to the recombination of hole and electrons, and allows holes andelectrons to be efficiently recombined, thus the light emissionefficiency can be improved. With a view to improving the electrontransportation efficiency and the hole transportation efficiency, theorganic ligands 21 are preferably placed on the surface of each quantumdot 20 as illustrated in FIG. 5A.

The light emitting devices of the embodiments shown in FIG. 8A to FIG.9C are conventional EL devices, and the light emitting devices of theembodiments in FIGS. 4A and 4B and FIGS. 6A to 6C have layeredstructures opposite to that of the Conventional EL devices. In the lightemitting devices of the embodiments shown in FIG. 8A to FIG. 9C, thethin film transistor is preferably a p-ch TFT; accordingly, the channellayer is preferably formed from a P-type semiconductor.

In this embodiment, at least one of the layer between the cathode 15 andthe emitting layer 13, the emitting layer 13, and the layer between theemitting layer 13 and the anode 11 can be formed by the inkjet process.As illustrated in FIG. 11 , a mask 30 is placed on the substrate 10, andan inorganic layer 31 is printed in a plurality of application regions30 a provided in the mask 30 by the inkjet process. Here, the surfacesof the sidewalls 30 b of the rises of the mask 30 are, for example,subjected to fluorination to impart water repellency to the sidewalls 30b. This can reduce the affinity of the surface of the sidewalls 30 b forink and prevents defects such as dents in the surface of the printedinorganic layer 31, and thus can increase the flatness of the surface ofthe inorganic layer 31.

This embodiment involves top emission devices, and can improve thecarrier balance in the inverted EL light emitting devices 3 depicted inFIGS. 4A and 4B and FIGS. 6A to 6C. In addition, the layers between thecathode 15 and the light emitting layer 13 (the electron transport layer14 or both the electron transport layer 14 and the electron injectionlayer 18) and the light emitting layer 13 can be formed by coating.Further, the layers between the light emitting layer 13 and the anode 11(the hole transport layer 12 or both the hole transport layer 12 and thehole injection layer 16) can be formed by vapor deposition or coating.This can facilitate the production process of the light emittingdevices.

The display device 1 depicted in FIG. 1 is an example, and thearrangement of the red emission regions 2 a, the green emission regions2 b, and the blue emission regions 2 c may be different from that inFIG. 1 . Further, of the red emission regions 2 a, the green emissionregions 2 b, and the blue emission regions 2 c, only the emissionregions of one color or the emission regions of two colors may beincluded in the display device.

As in this embodiment, in a display device using quantum dots, thequantum dots can be used to build either a point light source or asurface light source, and a curved light source or a flexible productmay also be obtained by selecting a suitable substrate.

Further, according to this embodiment, distinctive products such aslightings producing a mixture of colors comparable to that of sunlightwhich has been hardly obtained, lightings producing light easy on theeyes, and lightings optimized for plant factories can be developed.

Thus, display devices using quantum dots provide a high degree offlexibility in the design; for example, the devices can be formed to bethin, lightweight, and curved. Further, the devices can produce naturallight not dazzling in the eyes that produces less shadows. In addition,the devices consume less power and have a long life. For example,display devices using quantum dots of this embodiment are superior toorganic EL display devices in terms of color rendering properties,emission properties, product life, and product price.

A display device using quantum dots of this embodiment can be used as aPL emitter as well as an EL emitter. Further, for a display device usingquantum dots, a hybrid light emitting device in which an EL emitter anda PL emitter are stacked can be obtained. For example, a PL emitter issuperposed on a surface of an EL emitter, and the emission wavelength ofthe light emitted by excited quantum dots in the EL emitter can bechanged using the quantum dots contained in the PL emitter. The ELemitter is a light emitting device having a layered structure describedabove, and the PL emitter is, for example, a sheet-like wavelengthconverting member in which a plurality of quantum dots are dispersed ina resin. Such a hybrid structure can be obtained with the use of quantumdots.

Note that in this embodiment, in order to both increase the area of thedisplay device using quantum dots and reduce the production cost, theinkjet printing process, the spin coating process, or the dispensingprocess is preferably used as a method for applying the quantum dots.

EXAMPLES

The effects of the present invention will be described using Examples ofthe present invention. Note that the embodiments of the presentinvention are not limited to the following examples in any way.

The samples shown in Table 1 below were prepared to investigate the dropcharacteristics in the inkjet process. Note that in Table 1, “Abs10”refers to a sample exhibiting an absorbance of 10% with the quantum dotsbeing dispersed, and “Abs20” refers to a sample exhibiting an absorbanceof 20% with the quantum dots being dispersed.

TABLE 1 Viscosity R.T. Ejection Mass Mass/drop Sample Solvent SG (mPa ·s) (° C.) number (g) (g) Green QD Abs10 Cyclododecene 0.87 23.5 10million 0.0407 4.07E−09 Red QD Abs20 Cyclododecene 0.87 25.6 10 million0.0397 3.97E−09 Red QD Abs20 Tetradecane 0.77 26.1 10 million 0.03033.03E−09 Green QD Abs10 Tetradecane 10 million Cancelled Green QD Abs10Octadecene 0.79 3.12 25.0 10 million 0.0447 4.47E−09 Red QD Abs20Octadecene 0.79 2.97 26.7 10 million 0.0449 4.49E−09 PolyvinylcarbazoleDimethoxybenzene 1.08 26.7 10 million 0.0579 5.79E−09 PolyvinylcarbazoleDimethoxybenzene:Cyclo- hexylbenzene 1:1 PolyvinylcarbazoleDimethoxybenzene:Cyclo- 1.13 26.5 10 million 0.0527 5.27E−09hexylbenzene 2:1 Zinc oxide IPA:Propylene nanoparticles glycol 1:1Solvent only Tetradecane 0.77 1.60 23.8  1 million 0.0040 4.00E−09Tetradecane  3 million 0.0126 4.20E−09 Tetradecane 0.0109 3.63E−09Tetradecane 10 million 0.0362 3.62E−09 Solvent only ODE(1-Octadecene)0.79 3.10 23.0 10 million 0.0385 3.85E−09 Solvent onlyDecahydronaphthalene 0.88 2.20 23.0 10 million 0.0430 4.30E−09 Solventonly Cyclododecene 0.87 4.20 23.0 10 million 0.0514 5.14E−09 Solventonly Ethylene glycol 1.10 20.50 21.7 Failed — — Solvent only Ethyleneglycol 1:IPA 1 0.94 7.40 25.0 10 million 0.0500 5.00E−09 Solvent onlyEthylene glycol 3:IPA 1 1.02 12.80 22.7 10 million 0.0481 4.81E−09Solvent only Ethylene glycol 3:IPA 2 0.97 10.30 22.3 10 million 0.05005.00E−09 Solvent only Phenylcyclohexane 0.94 2.29 26.3 10 million 0.04444.44E−09 Solvent only Dichlorobenzene 1.30 1.62 25.5 Failed — — Solventonly n-Octane 0.70 Unmeasurable 24.3 Failed — — Volume/drop Adverseeffect Sample Solvent (pL) Drop on EPDM Notes Green QD Abs10Cyclododecene 4.68 + Red QD Abs20 Cyclododecene 4.56 + Red QD Abs20Tetradecane 3.94 + Cap deformation Head clogged Green QD Abs10Tetradecane + Green QD Abs10 Octadecene 5.66 + Red QD Abs20 Octadecene5.68 + Previous head replacement Polyvinylcarbazole Dimethoxybenzene5.34 + Head replacement Clogging after twice ethanol cleaning/ Cloggingof damper filter Polyvinylcarbazole Dimethoxybenzene:Cyclo- −Malformation of hexylbenzene 1:1 drops (2 drops) PolyvinylcarbazoleDimethoxybenzene:Cyclo- 4.66 + hexylbenzene 2:1 Zinc oxide IPA:Propylene− Defective drops nanoparticles glycol 1:1 Possibly high viscositySolvent only Tetradecane 4.60 + Tetradecane 4.83 + Tetradecane 4.18 +Tetradecane 4.16 + Solvent only ODE(1-Octadecene) 4.87 + Cap deformationSolvent only Decahydronaphthalene 4.89 + Affected Solvent onlyCyclododecene 5.91 + Solvent only Ethylene glycol — − Not affectedSolvent only Ethylene glycol 1:IPA 1 5.32 + Not affected IPA specificgravity: 0.78 Solvent only Ethylene glycol 3:IPA 1 4.72 + Not affectedSolvent only Ethylene glycol 3:IPA 2 5.15 + Not affected Solvent onlyPhenylcyclohexane 4.72 + Solvent only Dichlorobenzene — − AffectedSolvent only n-Octane — − Cap deformation

The “+” signs shown in the “drop” column in Table 1 correspond to thesamples that had been appropriately dropped and the “−” signs correspondto the samples that were failed to be appropriately dropped.

In Table 1, the samples of “Red QD” and “Green QD” are used in emittinglayers. Further, the samples of “polyvinylcarbazole” are used in holeinjection layers. The sample of “Zinc oxide nanoparticles” is used in anelectron transport layer or an electron injection layer.

Table 1 shows that IPA and propylene glycol were not preferred as thesolvent for zinc oxide nanoparticles, and another solvent had to beused. The solvents corresponding to the “+” signs in the “drop” columnshown in Table 1 can be appropriately used; however, hydrophilicsolvents are preferred. For example, as a hydrophilic solvent, analcohol-based solvent can be used.

FIG. 12 is a photograph showing a state where the application wasperformed by the inkjet process using ZnO_(X) dissolved inethoxyethanol:EG=7:3 as a solvent. As shown in FIG. 12 , a goodapplication state was achieved.

Further, adverse effects on ethylene propylene diene monomer rubber(EPDM) inside an inkjet head was also investigated. As shown in Table 1,in some samples, cap deformation occurred or EPDM was adverselyaffected. This demonstrated that the effect on EPDM was preferablyconsidered in the case of using EPDM.

(Experiment on Shell Thickness Dependence of Quantum Dot)

In this experiment, quantum dots (green QDs) of the samples shown inTable 2 were prepared. For a bottom emission display device includingthe light emitting device using the quantum dots in FIG. 4A, therelationship between the shell thickness and the external quantumefficiency (EQE) was investigated.

TABLE 2 Shell total CV EQE QY QD layer No. Core Coating 1 Coaring 2Coating 3 (nm) (%) (%) (%) FWHM thickness 1 GC_A I_A II_A III_A Thick 11.8 17.2 — 86 38 — 2 GC_A I_A II_A III_A Thick 2 2.4 18.8 4.5 80 39 37 3GC_A I_A II_A III_A Thick 3 3.1 15.2 0.9 43 39 23 4 GC_A I_A II_A — 1.212.4 0.5 85 30 15 5 GC_A I_A II_B — 1.8 21.9 2 88 32 25 6 GC_A I_A II_C— 2 18.9 1.2 82 38 27 7 GC_A — — — Reference 10.3 — 13 30 8 GC_A I_BThin — — 0.2 16.8 10 38 9 GC_A I_B Thin — — 0.9 12.5 91 34 10 GC_A I_B —— 1.3 12.9 6 86 33 36 11 GC_A I_B II_D — 1.4 14.5 6 93 33 31

As shown in Table 2, a correlation was found between the shell thicknessand the EQE. The shell thickness was, but not limited to, 0.1 nm or moreand 4.0 nm or less, preferably 0.5 nm or more and 3.5 nm or less, morepreferably 1.0 nm or more and 3.0 nm or less, still more preferably 1.3nm or more and 2.5 nm or less.

Further, the relationship between the quantum dot (QD) layer thicknessand the EQE was investigated, and the EQE was found to tend to be higherwhen the quantum (QD) layer had a certain level of thickness. Thequantum dot (QD) layer thickness was, but not limited to, 5 nm or moreand 50 nm or less, preferably 10 nm or more and 45 nm or less, morepreferably 15 nm or more and 40 nm or less, still more preferably 20 nmor more and 40 nm or less, yet more preferably 25 nm or more and 40 nmor less.

Further, Cd-based green quantum dots were subjected to PYS measurements.In FIG. 13 , the circle marks represent experimental data of the quantumdots each constituted by a core alone in Example 1, and the square marksrepresent experimental data of the quantum dots constituted by a corecoated with a shell in Example 2.

The photo-electron yield spectroscopy (PYS) can measure the ionizationpotential. For example, the measurement can be performed using a systemnamed AC-2/AC-3 manufactured by RIKEN KEIKI CO., LTD.

As shown in FIG. 13 , the energy of the rising edge in Example 1 wasfound to be different from that in Example 2. The energy rose atapproximately 6.1 eV in Example 1, and the energy rose at approximately7.1 eV in Example 2.

FIG. 14 shows the PYS measurement results of the Cd-based green quantumdots with different shell thicknesses in Example 3 and Example 4. Theshell thickness was larger in Example 4 than in Example 3. The energy ofthe rising edge in Example 3 was found to be different from that inExample 4. The energy rose at approximately 7.1 eV in Example 3, and theenergy rose at approximately 8.1 eV in Example 4.

(Experiment on Current Dependence of EQE)

FIG. 15 is an energy level diagram of each layer in the light emittingdevice used in an experiment. FIG. 16 is a graph illustrating therelationship between the current value and the EQE of an EL emitter anda PL emitter using red quantum dots; Further, FIG. 17 shows plotsillustrating the relationship between the current value and the EQE ofan EL emitter and a PL emitter using red quantum dots; and a plotillustrating the relationship between the current value and the EQE ofan EL emitter using blue quantum dots. The shell thicknesses in Example5 and Example 6 in FIG. 16 were different. The shell thickness waslarger in Example 5 than in Example 6. Further, in FIG. 17 , the shellthickness was largest in Example 7, and the shell thickness was smallerin Example 8 and Example 9 in this order.

As shown in FIG. 16 and FIG. 17 , for the EL emitter and the PL emitter,the EQE increased to the point around 20 mA. On the other hand, in thered emission device, the EQE increased even when the current value was20 mA or more. As shown in FIG. 16 and FIG. 17 , a larger shellthickness resulted in a larger increase in the EQE.

(Experiment on ZnO_(X) Synthesis)

FIG. 18 presents a graph illustrating the energy band gap Eg of eachlayer in the light emitting device used in an experiment, the energyE_(CB) at the bottom of the conduction band, and the energy E_(CB) atthe top of the conduction band, and an energy level diagram of thelayers. ZnO_(X)(Li) was used as L1 or L2 in FIG. 18 . Here, Li may ormay not be added. X is, but not limited to, around 0.8 to 1.2. As shownin FIG. 18 , it was found that use of ZnO_(X)(Li) as ZnO_(X) used forthe electron injection layer (ETL) and the electron transport layerincreased the band gap. ZnO_(X)(Li) is presumed to have an effect ofreducing the particle diameter. PVK shown in FIG. 18 corresponds to ahole injection layer; B1, B2, G(H), G(I3), and R(F) correspond toemitting layers (EL layers); and ZnO_(X), L2, and L4 correspond toelectron injection layers. When B1 or B2 is used for the emitting layer,ZnO_(X) can be used for the electron injection layer; however, it wasfound that when G(H), G(I3), or R(F) was used for the emitting layer, L2or L4 was preferably used for the electron injection layer. L2 and L4are ZnO_(X)(Li).

In particular, when an emitting layer (EL layer) with a shallowconduction band is used, it is advantageous to use ZnO_(X)(Li) for theelectron injection layer or the electron transport layer.

ZnO_(X)(Li) can be, but not exclusively, prepared by stirring a zincacetate-ethanol solution at a predetermined temperature for apredetermined time, and then mixing and stirring a LiOH.4H₂O-ethanolsolution, followed by centrifugal separation, cleaning, etc.

FIG. 19 to FIG. 21 show the UV (band gap), the PL spectra, and the PYSdata of ZnO_(X)(Li) and ZnO_(X)(K) used for the electron injection layer(ETL). ZnO_(X)(K) was produced using KOH catalytically and was not dopedwith K or Li. ZnO_(X)(Li) and ZnO_(X)(K) were found to show differencesin the UV data and the PL data. On the other hand, the PYS data ofZnO_(X)(Li) and ZnO_(X)(K) hardly differed, and the rising edge energieswere almost the same.

Thus, ZnO_(X) of which band gap is controlled by selecting fromdifferent particle diameters to be used for an electroninjection/transport layer of an EL device using quantum dots, and dopedZnO_(X) for which defects are controlled and the band gap is controlledby adding doping species can be proposed.

When the recombination of electrons and holes for producing light cannotstill be brought into balance, hole blocking functions are preferablyadded by interposing a thick insulating layer between the EL layer andthe electron injection layer or integrating ZnO_(X) with moleculesthereby adjusting the balance. Here, the integrated layer involves theintegration of, for example, ZnO_(X) with T2T(2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine). X is, but not limited to,around 0.8 to 1.2.

Further, ZnO_(X) is known to have functions suitable for use in not onlythe electron injection/transport layer but also in a holeinjection/transportation layer by performing ozone treatment or thelike. Specifically, performing ozone treatment on ZnO_(X) was found toimprove the hole transporting performance.

INDUSTRIAL APPLICABILITY

According to the present invention, a light emitting device containingquantum dots can be used for a display device, and excellent emissionproperties can be obtained.

This application is based on Japanese patent application No. 2017-215801filed on Nov. 8, 2017, the content of which is hereby incorporated inits entirety.

The invention claimed is:
 1. A display device, comprising: a displayarea, wherein the display area includes a light emitting device in whicha first electrode, a first layer between the first electrode and anemitting layer, the emitting layer, a second layer between the emittinglayer and a second electrode, and the second electrode are stacked inthis order on a substrate, wherein the emitting layer is formed of aninorganic layer containing quantum dots, and the light emitting deviceis a top emission device, wherein the quantum dots have a core-shellstructure with a shell thickness from 0.2 nm to 3.1 nm, wherein anelectron injection layer is provided on the first layer, which isbetween the first electrode and the emitting layer, or on the secondlayer, which is between the emitting layer and the second electrode,wherein the quantum dots are green quantum dots or red quantum dots, andwherein the electron injection layer comprises ZnO_(x)(Li), with x beingfrom 0.8 to 1.2.
 2. The display device according to claim 1, wherein athin film transistor is connected to the light emitting device, with thethin film transistor being an n-ch TFT.
 3. The display device accordingto claim 2, wherein an oxide semiconductor of the thin film transistoris an In—Ga—Zn—O-based semiconductor.
 4. The display device according toclaim 1, wherein the display device is flexible.
 5. The display deviceaccording to claim 1, wherein the quantum dots have a structure in whicha surface of a core is not covered by a shell.
 6. The display deviceaccording to claim 1, wherein at least one of the first layer betweenthe first electrode and the emitting layer, the emitting layer, and thesecond layer between the emitting layer and the second electrode isformed by an inkjet process.
 7. The display device according to claim 1,wherein the first layer between the first electrode and the emittinglayer, and the emitting layer are formed by coating; and the secondlayer between the emitting layer and the second electrode is formed byvapor deposition or coating.
 8. The display device according to claim 1,wherein the shell thickness is from 1.2 nm to 3.1 nm.
 9. The displaydevice according to claim 1, wherein the emitting layer is a quantum dotlayer formed by coating quantum dots, and wherein a thickness of thequantum dot layer is from 15 nm to 37 nm.